Semiconductor devices typically include a semiconductor substrate and one or more thin-films deposited onto the semiconductor substrate. The thin-films form layers that provide the basis for the electrical and optical characteristics of the semiconductor device. The process of forming these layers on the surfaces of a substrate is called thin-film deposition or simply deposition.
The term “dielectric layer” broadly refers to thin-films that are deposited to provide electrical insulation. An exemplary dielectric layer is a transistor gate dielectric film. A transistor gate dielectric film may be used, for example, to electrically insulate a gate electrode from an underlying substrate in a field effect transistor (FET). Nitrogen is typically incorporated into a dielectric layer to enhance the performance of the layer.
Nitridation refers to the process during which nitrogen is incorporated into a dielectric layer. Nitrogen incorporation, if properly performed, suppresses dopant diffusion, mitigates positive flatband voltage shifts, reduces leakage current, and improves reliability. Incorporated nitrogen, however, may not only fail to provide these benefits but also can significantly degrade carrier mobility, if the nitrogen is present in excess or placed at unsuitable locations in the dielectric layer.
In conventional dielectric layer formation, nitridation is applied either before or after the dielectric layer is deposited. The former process is called pre-deposition nitridation and the latter process is called post-deposition nitridation. In post-deposition nitridation, the dielectric layer is deposited to its final thickness and then nitrided, for example, by exposing the film to ammonia (NH3) gas at elevated temperatures (e.g., 300-500° C.) or by exposing the film to a remote plasma for nitridation. The entire dielectric layer is exposed to the nitridation process because the dielectric layer has already been deposited to its final thickness.
Pre-deposition nitridation typically involves applying a thin-film that is rich in N (e.g., a thin nitride film) to an upper surface of a substrate prior to depositing the dielectric layer. The dielectric layer is then deposited over the thin nitride film. In contrast to post-deposition nitridation, the nitrogen species are concentrated between the substrate and the dielectric layer. Examples of nitride films that are commonly used in pre-deposition nitridation are silicon nitride (Si3N4) and silicon oxynitride (SiOxNy).
Thus, conventional dielectric layer processing provides two extreme nitridation choices. Nitrogen species are either distributed throughout a dielectric layer in a substantially uniform concentration (e.g., with post-deposition nitridation) or nitrogen species are concentrated at the interface between the dielectric layer and an underlying substrate (e.g., in pre-deposition nitridation).